Optimal output filter design for grid-tied inverters with GaN-based switching devices

Abstract

Renewable energy resources are utilized in distribution networks based on an Active Front End (AFE) technology as a bidirectional power flow energy conversion system. Low and high frequency harmonics generated by switching pattern of the power electronics converter should be reduced by an appropriate output filter. According to international regulations, harmonics injected to the grid by Pulse Width Modulated (PWM) power converters must be handled to maintain power quality indices within standard limits. LCL filters with passive damping resistors are the most reliable and renowned devices to fulfil the standards. The main objectives in the design of an efficient LCL filter are to reduce the cost and weight of the filter, as well as to increase the robustness and stability of the power electronics converter. Accurate formulas and methods are vital to design an optimum filter. Hence, in this thesis, a comprehensive and mathematical approach is proposed to calculate the maximum current ripple of a PWM-based inverter, which helps to precisely calculate the size of the inverter-side inductors in the LCL filter. The mathematical approach to solve the problem of optimal damping resistor selection for output filters is presented, which can be implemented analytically in different configurations of the LCL filters. The proposed method also works to solve an optimal damping problem for any type of filter.Multi-level inverters are outstanding solutions to significantly reduce the voltage stress and harmonics created by PWMs. Filter design for grid-tied multi-level inverters is quite a new topic in power electronics. In this research, novel mathematical expressions are derived to calculate the maximum ripple of the output current of a cascaded multi-level inverter, which are used to size the inverter-side inductor for an output filter. Moreover, a mathematical expression for the grid-side inductor is attained considering compatibility level for harmonics within the range of 2-150 kHz to cover electrical and electronics equipment, which currently are the most important issues in the international standardization committee (IEC, TC77A). Additionally, active power losses across the damping resistor of the filter for a cascaded multi-level inverter is analytically calculated to consider as a constraint during the design of the damping resistor. Based on these novel mathematical expressions, a filter design approach is proposed for cascaded multi-level inverters.Finally, a unified optimization-based filter design approach for a grid-tied inverter is introduced. The proposed filter design approach is based on finding the precise maximum ripple current according to the modulation method and calculating the accurate optimal passive damping resistor. Unique formulas for the calculation of active power losses are introduced, which consider all the influential design parameters. Afterwards, a new optimization-based algorithm is presented to determine the optimal size of the inductors and capacitor considering constraints on damping losses. The simulation and experimental results prove the efficiency of the proposed optimization-based method as well as the accuracy of the introduced filter parameters expressions. The experimental setup consists of an inverter which uses GaN-based switches in its bridges, and a real time micro-controller is used to control the inverter switching process.The main contributions of this research to achieve the aforementioned aims can be summarized as follows:· Proposing a comprehensive solution to calculate the maximum current ripple of PWM-based inverters.· Solving the problem of optimal damping selection to minimize the peak of the LCL-filters transfer function.· Proposing a complete solution to calculate the maximum current ripple of Cascaded Multi-level Inverters (CMI).· Proposing a novel compact formula to calculate the maximum active power losses across the damping resistor of the LCL filters for CMIs.· Calculation of the damping resistor of CMIs based on both stability and efficiency of the filter.· Proposing a very fast numerical method to calculate the maximum inverter current ripple of LCL filters.· Proposing closed-form and accurate expressions to calculate the exact value of the damping active power losses of different configurations of LCL filters.· Proposing a new optimization-based method to design the LCL filters by considering the size of the filter as the main objective.· Building GaN-based 2-level and multi-level inverters in the lab and validation of the proposed design by performing practical experiments.